There is currently a great need to expand the knowledge base of various functional capacity measures used in the rehabilitation of chronic low back pain (CLBP) patients. In the current study, the functional performance of lifting capacity differences among four separate CLBP patient groups were examined by using standardized isokinetic and isoinertial lifting performance measurements. One hundred ninety-three consecutive patients were assessed at three separate points in time: at initial evaluation (PRE), at admission to the intensive 3-week phase of a functional restoration program (ADM), and after program discharge and follow up (FU) (an average of 12 weeks later). Group 1 (n = 26) consisted of postdiscectomy CLBP men; Group 2 (n = 91) consisted of nonsurgical CLBP men; Group 3 (n = 17) consisted of postdiscectomy women; and Group 4 (n = 59) consisted of nonsurgical women. Results comparing admission scores to postdischarge scores revealed that all four groups demonstrated a significant increase (P < 0.01) in Liftask and progressive isoinertial lifting evaluation performance. Increases in performance were virtually identical for nonoperative and postoperative patients, with postdiscectomy men and women actually achieving peak isokinetic lifting forces higher than unoperated patients at the program's conclusion. This achievement in human performance was not found for the progressive isoinertial lifting evaluation test. Overall, these findings illustrate the utility of quantitative functional capacity measures, and provides objective evidence of the gains achieved by the patients.(ABSTRACT TRUNCATED AT 250 WORDS)